Understanding the Optoelectronic Properties of Crown-Ether Perovskites and Their Compositionally Substituted Mixed-Halide Derivatives

Insertion of crown-ethers into ordered-vacancy perovskites increases the distance between octahedral units, flattening their bands due to enhanced quantum confinement making these systems behave as separate units, thus promoting an experimentally near-unit photoluminescence quantum yield. To unveil how X-site mixing modulates the optoelectronic band gaps, i.e., the electronic band gap and exciton binding energies, of supramolecular zero-dimensional perovskites, we address the recently synthesized family of (18C6@A)2BX6 perovskites by studying their (18C6@K)2B(XxX1−x′)6 with B = Hf and Zr; X = Cl, Br, and I mixed-halide derivatives to find out that the halide mixing induces large bowing band gap parameters that follow the trend of the atomic radii of the X-site-substituted chemical species. The isolated inorganic octahedra in the structure define dispersionless bands. These characteristics in these materials led us to develop an exciton model that assumes that the exciton binding energy is led by the arrangement of partial charges and is highly localized within the octahedral units of the system with a low discrepancy compared to experiment. Hence, halide mixing also triggers large exciton binding energy bowing, while relative atomic displacements as key factors for excitonic energies shifts. Hence, our results provide concrete guidelines for engineering band gaps and exciton binding energies in crown-ether supramolecular perovskites, enabling the targeted design of efficient emitters and wavelength-tunable optoelectronic devices.